1. Field of the Invention
The present invention relates to a calibration method for a lithographic apparatus and to a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern on an impinging light beam.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus, a substrate table will generally be movable and its position within a coordinate system can be known and controlled with very great accuracy. For example, this can be done using interferometric measuring means. Similarly, if an array of individually controllable elements is movable, then its position within a coordinate system, possibly a different coordinate system, will also be measurable very accurately. The coordinate of any addressable pixel within the array of individually controllable elements will be very accurately defined. However, although arrays of individually controllable elements are used in lithographic projection apparatus, there are not known calibration methods for determining the relationship between the coordinate system of the array of individually controllable elements and the coordinate system of the substrate table. This calibration is desired to image a feature at a specific location on a substrate on the substrate table so that it is known where to generate that feature on the array of individually controllable elements.
Without properly defined calibration methods there exists further problems, such as the calibration might take an excessively long length of time, for example, because certain arrays of individually controllable elements consist of tens of millions of movable mirrors which would have to be calibrated. If the calibration is not properly performed, there is the problem that defective patterns will result. A further problem regarding calibration is that in some lithographic projection apparatus the wafer table and/or array of individually controllable elements are scanned, such that they are in motion during the exposure of the pattern and the radiation system may be pulsed. This means that, as well as spatial calibration, it is also necessary to calibrate the timing of the radiation pulses with respect to the velocities of the movable components, because there can be a delay between a trigger signal for the pulse of radiation and the actual emission of the radiation pulse.
It is an object of the present invention to provide a calibration method for determining a relationship between a coordinate system of an array of individually controllable elements and a coordinate system of a substrate table.